1,112 research outputs found
String Gas Baryogenesis
We describe a possible realization of the spontaneous baryogenesis mechanism
in the context of extra-dimensional string cosmology and specifically in the
string gas scenario.Comment: arXiv admin note: substantial text overlap with 0808.0746 by
different autho
Vector Perturbations in a Contracting Universe
In this note we show that vector perturbations exhibit growing mode solutions
in a contracting Universe, such as the contracting phase of the Pre Big Bang or
the Cyclic/Ekpyrotic models of the Universe. This is not a gauge artifact and
will in general lead to the breakdown of perturbation theory -- a severe
problem that has to be addressed in any bouncing model. We also comment on the
possibility of explaining, by means of primordial vector perturbations, the
existence of the observed large scale magnetic fields. This is possible since
they can be seeded by vorticity.Comment: v3. Two reference added; Identical with version accepted for
publication at PR
Wrapped brane gas as a candidate for Dark Matter
We consider brane gas models based on type II string theories and analyze the
mass, the Ramond-Ramond charge and the charge on moduli fluctuations of branes
wrapping over cycles of a compactified space in the four-dimensional Einstein
frame. A six-dimensional torus and Calabi-Yau threefolds are considered for the
Kaluza-Klein reduction. A large volume of the compactified space and a weak
string coupling gives rise to point particles of the wrapped branes which have
a light mass and a small charge of the Ramond-Ramond flux and of the moduli
fluctuations, while the particles become very heavy in the string frame. We
find that the masses and the charges satisfy the sea-saw like dual relations
which become time-independent in the four-dimensional Einstein frame.Comment: 17 pages, 2 figures, revtex4, v3: comments adde
Can universe exit from phantom inflation due to gravitational back reaction?
The effects of the gravitational back reaction of cosmological perturbations
are investigated in a phantom inflation model. The effective energy-momentum
tensor of the gravitational back reaction of cosmological perturbations whose
wavelengths are larger than the Hubble radius is calculated. Our results show
that the effects of gravitational back reaction will counteract that of the
phantom energy. It is demonstrated in a chaotic phantom inflation model that if
the phantom field at the end of inflation is larger than a critical value
determined by the necessary e-folds, the phantom inflation phase might be
terminated by the gravitational back reaction.Comment: 9 pages, Revtex4, to appear in JCA
Non-Thermal Production of WIMPs and the Sub-Galactic Structure of the Universe
There is increasing evidence that conventional cold dark matter (CDM) models
lead to conflicts between observations and numerical simulations of dark matter
halos on sub-galactic scales. Spergel and Steinhardt showed that if the CDM is
strongly self-interacting, then the conflicts disappear. However, the
assumption of strong self-interaction would rule out the favored candidates for
CDM, namely weakly interacting massive particles (WIMPs), such as the
neutralino. In this paper we propose a mechanism of non-thermal production of
WIMPs and study its implications on the power spectrum. We find that the
non-vanishing velocity of the WIMPs suppresses the power spectrum on small
scales compared to what it obtained in the conventional CDM model. Our results
show that, in this context, WIMPs as candidates for dark matter can work well
both on large scales and on sub-galactic scales.Comment: 6 pages, 2 figures; typo corrected; to appear in PR
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
Dynamical Relaxation of the Cosmological Constant and Matter Creation in the Universe
In this Letter we discuss the issues of the graceful exit from inflation and
of matter creation in the context of a recent scenario \cite{RHBrev} in which
the back-reaction of long wavelength cosmological perturbations induces a
negative contribution to the cosmological constant and leads to a dynamical
relaxation of the bare cosmological constant. The initially large cosmological
constant gives rise to primordial inflation, during which cosmological
perturbations are stretched beyond the Hubble radius. The cumulative effect of
the long wavelength fluctuations back-reacts on the background geometry in a
form which corresponds to the addition of a negative effective cosmological
constant to the energy-momentum tensor. In the absence of an effective scalar
field driving inflation, whose decay can reheat the Universe, the challenge is
to find a mechanism which produces matter at the end of the relaxation process.
In this Letter, we point out that the decay of a condensate representing the
order parameter for a ``flat'' direction in the field theory moduli space can
naturally provide a matter generation mechanism. The order parameter is
displaced from its vacuum value by thermal or quantum fluctuations, it is
frozen until the Hubble constant drops to a sufficiently low value, and then
begins to oscillate about its ground state. During the period of oscillation it
can decay into Standard Model particles similar to how the inflaton decays in
scalar-field-driven models of inflation.Comment: 6 page
Single Field Baryogenesis
We propose a new variant of the Affleck-Dine baryogenesis mechanism in which
a rolling scalar field couples directly to left- and right-handed neutrinos,
generating a Dirac mass term through neutrino Yukawa interactions. In this
setup, there are no explicitly CP violating couplings in the Lagrangian. The
rolling scalar field is also taken to be uncharged under the quantum
numbers. During the phase of rolling, scalar field decays generate a
non-vanishing number density of left-handed neutrinos, which then induce a net
baryon number density via electroweak sphaleron transitions.Comment: 4 pages, LaTe
On T-Duality in Brane Gas Cosmology
In the context of homogeneous and isotropic superstring cosmology, the
T-duality symmetry of string theory has been used to argue that for a
background space-time described by dilaton gravity with strings as matter
sources, the cosmological evolution of the Universe will be nonsingular. In
this Letter we discuss how T-duality extends to brane gas cosmology, an
approximation in which the background space-time is again described by dilaton
gravity with a gas of branes as a matter source. We conclude that the arguments
for nonsingular cosmological evolution remain valid.Comment: 8 pages, Appendix adde
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